The effect of heterogeneity on the flow behavior on high density polyethylene

The effect of heterogeneity on the flow behavior of high density polyethylene (HDPE) has been studied by systematically homogenizing heterogeneous blends and observing the effect on the basic rheological parameters and on the extrudate surface appearance. Ample evidence is presented to show that heterogeneity causes substantial changes in the flow behavior although molecular parameters obtained through solution studies are unchanged. A simplified mathematical model which clearly illustrates the effects of the size of these flow units on the flow behavior is presented.     

Stress-Cracking of high density polyethylene in detergents

Correlations of the stress intensity factor, K_I, with crack speed, ?, have been obtained in a number of detergent solutions each having different detergent concentration. A constant crack speed region was observed in high density polyethylene. The K_I independent constant crack speed was found to vary linearly with detergent concentration. The viscosity of the detergent solution increases with concentration and hence this region is not controlled by the hydrodynamic properties of the environment in contrast with Williams' model. The K_I-? data were compared with existing models of crack propagation.     

Impact yielding of high density polyethylene

We have used a projectile impact method to estimate the flow stress of high density polyethylene at a strain rate of 3 × 10³ sec⁻¹. The technique was developed initially by Taylor and applied successfully by Whiffin and others to ductile metals. The data from this experiment have been compared with data obtained in more conventional compression and drop hammer tests at lower strain rates at 20° and 100°C. The flow stress of high density polyethylene deduced from the impact test at 20°C is significantly higher than that anticipated from a simple extrapolation of the low strain rate data at 20°C. The data at 100°C are however in good agreement. The technique has also been used to estimate the flow stress of high density polyethylene as a function of temperature over the range −20° to +105°C. These data indicate that the discrepancy in the data for 20°C arises from a real discontinuity in the response of the polymer rather than from an inadequacy in the theoretical analysis of the impact experiment as applied to polymeric solids. We conclude that the impact method described is a useful technique for estimating the flow stress of polymers. It is however limited to a relatively narrow range of strain rates.     

Inhomogeneous deformation in welded high density polyethylene

The effect of the thermal history on the properties of welded high density polyethylene is studied. The lamellar microstructure observed in the weld is different from that in the bulk slow cooled material. The weld has lower crystallinity and smaller lamellar size, both of which change with annealing. The differences in the microstructure between the weld and the surrounding material produce differences in the plastic properties. The low ductility of the welded samples is a direct result of the relatively low yield stress within the weld. Annealing of the weld can improve the tensile properties, but in quenched welds the final properties are still inferior to the bulk material. Displacement controlled, constant velocity microindentation tests are shown to provide a rapid means of evaluating the properties of the weld. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 43–52, 2002     

Failure sensitivity of high density polyethylene in compression

As part of an investigation of the compressive mechanical behavior of high density polyethylene between room temperature and the crystalline melting temperature, the strain-to-failure as a function of temperature and strain rate was studied. The two resins studied in applied-strain-rate tests were found prone to fail, as judged by decreased strain-to-failure, at temperatures from 70 to 88°C. The strain-to-failure decreased as the temperature was increased or as the strain rate was decreased. This behavior is opposite to that observed in tension at lower temperatures. At temperatures just below melting, the strain-to-failure apparently began to increase again. By analogy to the results of tearing experiments on polyethylene and other thermoplastics, the findings are explained in terms of the influence of the α relaxation in polyethylene. Differences in the behavior of the two polyethylene resins were also examined.     

氯化聚乙烯专用HDPE树脂剖析

采用凝胶渗透色谱仪、差示扫描量热仪、扫描电子显微镜等考察了氯化聚乙烯(CPE)专用高密度聚乙烯(HDPE)的结构、性能和颗粒形态.结果表明: CPE专用HDPE具有中等相对分子质量、单峰相对分子质量分布、低共聚单体含量、高结晶度、高密度和低蜡含量等特点;进口料的粒径控制较好,粗颗粒含量更少;国产料具有更大的比表面积、孔容和更精细的形态结构.     

我国高密度聚乙烯进口情况分析

从高密度聚乙烯(HDPE)的进口量、贸易方式、进口来源和价格等方面分析了我国近几年HDPE的进口情况和变化趋势。我国HDPE的进口量呈快速增长态势。在不同贸易方式中,以一般贸易方式进口HDPE为主,且每月进口HDPE的量波动很大;从进口来源上看,中东国家对我国的出口量增长很快。我国进口HDPE品种全且牌号多,涉及到HDPE应用的各个方面;从中东和东南亚等低成本地区主要进口通用型牌号,而从欧洲、日本、韩国和美国等主要进口高附加值产品。最后,对我国HDPE生产企业提出了相关建议。     

Thermal and morphological evaluation of very low density polyethylene/high density polyethylene blends

Blends of very low density polyethylene (VLDPE) and high density polyethylene (HDPE) were prepared by melt extrusion. These blends exhibit a tendency to phase segregate when they are slow cooled from the melt. If they are cooled at increasingly faster rates, a finite population of co‐crystals can be isolated from the rest of the phase segregated material, indicating that this system is probably miscible in the melt but phase separates during cooling. Transmission electron microscopy observations are consistent with the blend melt miscibility since inter‐lamellar mixing was clearly appreciated in the samples examined. Other effects arising from interactions between the polymers were the nucleation of VLDPE rich phase by HDPE rich phase, and a melting point depression of HDPE rich phase caused by a dilution effect exerted by molten VLDPE rich phase. After a successive self‐nucleation and annealing thermal fractionation procedure is applied to the blends, phase separation dominates the behavior, although some small fraction of co‐crystals was still present.     

Microhardness of extruded high density polyethylene fibers

High-density polyethylene of high tensile modulus has been produced by solid state extrusion using an Instron capillary rheometer. Microhardness measurements on these ultraoriented fibers have been made to assess their perfection from values of the tensile elastic modulus and shear strength. The microhardness tests were measured using a Vickers square diamond. The microhardness increased with the common temperature for crystallization and extrusion, likely due to improvement in the lateral packing of microfibrils. The variation of microhardness with draw ratio is also illustrated.     

Annealing of ultra-oriented high density polyethylene extrudates

The annealing characteristics of highly-oriented, high density polyethylene (HDPE) fibers extruded at 90°C to constant extrusion draw ratios (EDR) of 5.8 to 30 have been studied using small-(SAXS) and wide-angle (WAXD) x-ray diffraction, differential scanning calorimetry (DSC), thermal shrinkage and tensile tests. The thermal stability in macroscopic properties, such as transparency, sample dimension, and tensile modulus, remarkably increases with the sample EDR. However, the folded chain crystals within microfibrils sensitively reorganize on annealing at ≥ 110°C, even when the macroscopic properties exhibit no significant changes at high EDR's. Although the EDR has a minor effect on the reorganizability of the fold chain crystals, it has a major influence on the SAXS intensity and the extended chain crystalline component detected by WAXD for both unannealed and annealed extrudates. This specific crystalline component has an improved thermal stability and enhances the thermal stability of macroscopic properties at higher EDR. The fibers extruded at 90°C in this study and those prepared at 134°C in a prior study, exhibit significantly different melting behavior after annealing and for long period vs. EDR relationships. These facts combined with the independent observations of the effect of annealing and extrusion temperatures on the consequent long period strongly suggest that annealing during extrusion plays an important role in determining the microstructure of extrudates, especially for extrusion near the ambient melting point of the polymer.     

Continuous extrusion of self-reinforced high density polyethylene

Continuous extrusion was studied of self-reinforced high density polyethylene (HDPE) sheets from flow-induced crystallization at die pressures varying from 30 to 60 MPa. Their morphology, thermal behavior, tensile strength, and light transmittance were tested. Flow fields of a polymer melt through a converging wedge channel were also investigated by direct visual observations in conjuction with a theoretical analysis. The extensional strain rate increased abruptly as the melt approached the exit of the converging channel, this resulting in a higher crystallization rate. So, achieving the crystallization of molecular chains just in front of the exit of the converging channel may favor to extrude the bulk polymeric materials having high properties under lower pressures (e.g., 40 MPa or lower), this having been realized in the present work. The tensile strength of the self-reinforced HDPE sheet prepared at a 40 MPa pressure was enhanced by a factor of 8.     

Morphology in thermally oxidized high density polyethylene pipes

In a continuation of previous work (1), the melting and crystallization behavior of the layered oxidized skin on thermally oxidized inner wall surfaces of different high density polyethylene (HDPE) pipes was studied. By hot stage polarized light microscopy on 5 μm thick cross-sections of the skin, melting was shown to proceed successively as a front moving inwards as temperature was raised. Analogously, crystallization of the skin layers proceeded with the front advancing outwards towards the skin surface at decreasing temperature. The kinetics were followed and the data was compared with previous DSC thermograms (1) on similar samples. The high temperature melting peak of the oxidized skin reported in earlier DSC-work (1) is shown to be associated with material with normal spherulitic texture.     

Capillary flow of low‐density polyethylene

The capillary flow of a commercial low‐density polyethylene (LDPE) melt was studied both experimentally and numerically. The excess pressure drop due to entry (Bagley correction), the compressibility, the effect of pressure on viscosity, and the possible slip effects on the capillary data analysis have been examined. Using a series of capillary dies having different diameters, D, and length‐to‐diameter L/D ratios, a full rheological characterization has been carried out, and the experimental data have been fitted both with a viscous model (Carreau‐Yasuda) and a viscoelastic one (the Kaye—Bernstein, Kearsley, Zapas/Papanastasiou, Scriven, Macosko, or K‐BKZ/PSM model). Particular emphasis has been given on the pressure‐dependence of viscosity, with a pressure‐dependent coefficient β_p. For the viscous model, the viscosity is a function of both temperature and pressure. For the viscoelastic K‐BKZ model, the time‐temperature shifting concept has been used for the non‐isothermal calculations, while the time–pressure shifting concept has been used to shift the relaxation moduli for the pressure‐dependence effect. It was found that only the viscoelastic simulations were capable of reproducing the experimental data well, while any viscous modeling always underestimates the pressures, especially at the higher apparent shear rates and L/D ratios. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Numerical simulation of crystallization in high density polyethylene extrudates

Three‐dimensional numerical simulation of solidification of high density polyethylene (HDPE) in a parallelepiped shaped extrudate is accomplished using a cell model for spherulite growth on a microscopic level under quiescent conditions and coupled with an enthalpy based heat transfer equation on the macroscopic level. The parallelepiped is cooled from the melting point of HDPE by a stream of air blown across it. When the thickness of the extrudate is of the order of microns, the distribution of the degree of crystallinity and temperature is uniform, and the lumped system formulation to model crystallization and heat flow is applicable. But significant variations inside the extrudate are observed when the thicknesses are of the order of millimeters and centimeters. The non‐dimensional Deborah and Biot numbers are shown to be important in the applicability of the formulation. The effects of air speed and ambient temperature on the crystallization process are also studied. It is observed that both (1) an increase in the air speed and (2) a decrease in the ambient temperature increase the rate of crystallization in the parallelepiped. The former has much greater effect than the latter in changing the average convective heat transfer coefficient. As a result, increasing the air speed results in much larger spherulites compared to reducing the ambient temperature. Decrease of temperatures in the extrudate during the either cooling processes is observed to be non‐monotonic owing to the release of the latent heat during crystallization. Introducing a constant temperature (melting point of HDPE) at a cross section of the extrudate changes the distribution of relative crystallinity and temperature in the extrudate significantly: the temperature gradient becomes much higher along the extrudate axis, and the material near the cross section never solidifies. Also, the extrudate behaves more like a fin as the variations in the thickness direction become insignificant.     

Mold replication in injection molding of high density polyethylene

In order to deepen the mechanisms at the basis of mold surface replication onto the molded plastic surface, a novel experimental approach is proposed. Up to 20 different mold surface textures were made by machining with repetitive patterns of peaks and valleys. Mold replication tests were performed by over-molding of high density polyethylene (HDPE) on steel inserts. The surface morphology of inserts and injection molded parts was acquired by surface analyzer, and all the main roughness parameters were extracted and compared as well as the geometrical profiles. Surface morphology was also measured on molded samples after thermal relaxation at 100°C. As expected, a strong correlation was found between the roughness of mold insert and molded part over the full experimented range. Profiles on the molded surface have the same repetitive pattern of the corresponding insert surface but with lower peaks, higher valleys, and a horizontal shrinkage. Comparing molded HDPE surface profiles before and after thermal relaxation, it was observed a similar change to the one highlighted between mold insert and molded part. This occurrence suggests that the final surface appearance of the molded part is also a function of the relaxation mechanism during or immediately after injection molding.     

Solubility and diffusivity of cyclohexane in high density polyethylene

The diffusivity and solubility of cyclohexane in a high density polyethylene, HDPE, were studied using a gravimetric, quartz‐spring, sorption balance. Solvent concentrations up to a weight fraction of 0.15 over a temperature range of 90 to 160°C were measured. Diffusion coefficients in the range of 10^?6 to 10^?7 were determined. Two types of polymer samples were used: a commercial bead form and flat sheets prepared by melting the polymer. Within the experimental error no differences were observed between the two forms indicating that there were no significant effects caused by the melting and compression. The solubility of cyclohexane in the HDPE as a function of the activity of the cyclohexane was linear. Above the melt temperature the solubility data were predicted better by the group‐contribution, lattice‐fluid equation of state (GCLF‐EoS) than by the van der Waals free‐volume (UNIFAC‐vdw‐FV) model. Below the melt temperature a correction factor for the elasticity significantly improved the predictions for both models. Although the HDPE has a crystallinity of 77.6%, the experimental data and the Vrentas‐Duda free‐volume theory indicated no significant tortuosity effects. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Some aspects of flame retardancy in high density polyethylene

The influence of the flame retardant agents, antimony trioxide and a chlorinated hydrocarbon, on the mechanical and flammability properties of high density polyethylene has been studied. Flammability was assessed by means of the limiting oxygen index test, whilst mechanical properties were measured in the tensile mode on compression moulded samples. An optimum in terms of flame resistance was found at a Sb:Cl mole ratio of 1:3 which tends to confirm that the actual flame retardant is the volatile antimony trichloride. The modulus, yield and drawing behaviour, and ultimate properties of the unoriented samples did not show significant change (< 10%) until the combined level of additive exceeded 25% by wt. Above this level it was found that the samples could not be oriented and that the elongation to break decreased markedly. Alumina trihydrate was studied as an alternative flame retardant but was found to be unsuitable for use in HDPE, since to obtain an adequate level of fire retardancy a high concentration (40%) of additive was required, which resulted in a significant deterioration in the mechanical properties.     

Kinetics of environmental stress cracking in high density polyethylene

The observation of environmental stress crack (ESC) growth in high density polyethylene (HDPE) in a 10% lgepal CO-630 solution is reported using double-edge- notched specimens, which allow a fracture mechanics approach. Below the initial stress intensity factor K₁ value of 0.4 MPa m^1/2, the cracking process consisted of both an incubation time for cracking, t_d′ and a crack growth stage. The incubation time is stress dependent (decreasing with increasing stress), while the crack growth exhibits a root time $\text{(t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ dependence and is relatively stress independent. The incubation time is the time necessary to generate a dry void craze structure sufficient to allow the PE to absorb the aggressive liquid. As a consequence of the liquid transport in the craze structure, the crack growth is believed to be controlled by the velocity of the liquid entering the void/fibril structure where capillary pressure is the driving force. The incubation times were determined to be more significant than the actual average crack growth rates for the PE samples tested. Injection moulding orientation increases the average crack growth rate without significantly changing the incubation time.     

High strength polyethylene fibers from high density polyethylene/organoclay composites

High strength fibers were prepared from high density polyethylene (HDPE)/organically modified montmorillonite (OMMT) composites. X‐ray diffraction study revealed that the composites were of conventional or phase‐separated type. As‐spun composite fibers were found to have higher drawability than as‐spun HDPE fiber. As a result of an increased drawability, fibers with much higher mechanical properties were obtained. The highest modulus and tensile strength obtained in the present study were 38 GPa and 1.7 GPa, respectively. Study of internal morphology suggests that the role of OMMT is to suppress a defect formation and allows the fiber to be drawn to higher draw ratio. Analysis of the mechanical properties of the fibers using a Griffith type relationship suggested that the fibers have much smaller defects and the predicted attainable strength for the fiber is much higher than that previously predicted for melt‐spun and hot drawn fiber. POLYM. ENG. SCI., 47:943–950, 2007. © 2007 Society of Plastics Engineers